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Abstract

We present a study of the optical properties of three-armed square nanospirals made of silver and realized as nanostructured thin films with Glancing Angle Deposition. Calculation of current flows in the nanospirals show excited resonant modes resembling those observed in U-shaped resonators. Four principal resonances were determined: near 200 THz and 480 THz for one polarization and 250 THz and 650 THz for the polarization orthogonal to the first one. In particular, a mode with anti-parallel current flow in opposite arms, associated with the observed resonance near 650 THz, indicates the existence of a magnetic-like resonance in the square nanospiral arrays. The robustness of the resonances against variations in the structural parameters of the nanospirals was investigated. This study revealed that the main parameter driving the position of the resonances was the overall dimension of the nanospiral, directly related to the length of their arms. Optical properties of a sample were measured by generalized spectroscopic ellipsometry at near-normal incidence, and evidence conversion between polarization states even for light polarized in the plane containing one of the arms in agreement with the numerical study. The measurements compared favorably to the results of the numerical simulations taking into account the disorder in the sample.

Figures (8)

Schematic of: (a) one nanospiral with the geometrical parameters used in side and (b) top views and (c) one unit cell of the dense array of nanospirals where one nanospiral has been highlighted. In (c) periodic boundaries were applied along x and y.

Absorption calculated for (a) x-polarized light and (b) y-polarized light and. The insets present snapshots of the instantaneous current distribution in one nanospiral at the positions of the resonances in absorption. The current intensities are encoded on a gray scale ranging from black to white for current intensity increasing. The arrows indicate the current directions.

Absorption spectra (a) for x-polarized light and (b) y-polarized light calculated for different diameter of the arms. (c) Coefficients of conversions of polarization. The length of the arms and their angle with the (x,y) plane were set to 200 nm and 24°, respectively.

Absorption spectra (a) for x-polarized light and (b) y-polarized light calculated for different length of the arms. (c) Coefficients of conversions of polarization. The diameter of the arms and their angle with the (x,y) plane were set to 64 nm and 24°, respectively.

Absorption spectra (a) for x-polarized light and (b) y-polarized light calculated for different angle of the arms with the (x,y) plane. (c) Coefficients of conversions of polarization. The length of the arms and their diameter were set to 64 nm and 203 nm, respectively.

Absorption spectra (a) for x-polarized light and (b) y-polarized light calculated in the periodic case (dotted line) and aperiodic case (full line). (c) Coefficients of conversions of polarization. The aperiodic case was obtained by averaging ten different random realizations. One realization consisted in random variations by ± 10% at most of the initial locations of all nanospirals.

Generalized ellipsometric parameters ψ and Δ presented as Jones matrix elements column (a) measured (line + dots) and column (b) calculated (full line) for incidence angle of 25°. The first quadrant of the first column depicts the top view of one nanospiral with the orientation of s-polarized light.